The antiphospholipid syndrome (APS) is a potentially lethal autoimmune clotting disorder that leads to thromboembolic events and obstetric complications and is frequently associated with lupus and other systemic autoimmune diseases. Treatment for APS is currently limited to life-long anticoagulation in order to prevent future thromboembolic events. The cause of APS is unknown but infectious triggers have been implicated in transient induction of antiphospholipid antibodies. The human gut microbiota exceeds 10-fold the number of the host eukaryotic cells, providing a major source for antigenic variation and persistent immune activation. This commensal variability has never been explored for the possibility of molecular mimicry in chronic autoimmune diseases. We hypothesize that commensals within the benign gut microbiota persistently induce pathogenic autoantibodies in genetically predisposed individuals via this mechanism. We have preliminary data to support a fundamental role of the gut microbiota in a murine model of APS. Importantly, we have identified potentially cross-reactive human commensals based on high sequence homologies with both the key T and B cell autoantigenic epitopes of the major autoantigen in APS (2-glycoprotein I; 2GPI) and cultured a candidate commensal. We plan to test the cross-reactive potential of autoreactive T and B cells from APS patients using synthetic peptides and cultured commensal protein extracts. To this end, we propose a study to collect peripheral blood and stool from anti-2GPI-positive APS and control patients longitudinally at three time points. We will define the autoantigenic epitopes targeted by CD4+ T and B cells from APS patients and design PCR- based strategies for targeted screening of the fecal microbiomes for the candidate commensals that carry homologous amino acid sequences to the autoepitopes targeted in these patients. We will also take the unbiased approach of high-throughput 16S rRNA sequencing of the entire fecal microbiome in order to discover previously unknown candidates that track with 2GPI immunoreactivities. This approach already revealed an additional candidate with cross-reactive potential. We propose that fluctuations of autoantigen- mimicking commensals will correlate with titers of anti- 2GPI antibodies in stool or blood and with autoantigen- specific T cells in APS patients. Finally, we have cloned 2GPI-specific CD4 memory T cells and will test + cross-reactivity with cultured key candidates and synthetic peptides. For candidates with both T and B cell epitope homologies, we will also test cross-reactivity of autoantibodies using ELISA and western blot. In summary, we aim to discover the persistent triggers of pathogenic autoantibody production in APS. These studies will represent a novel paradigm for how human autoimmunity can arise and will serve as the basis for development of entirely novel therapeutic avenues in systemic autoimmunity that are aimed at the gut microbiota.